Interferometric systems are suitable for, inter alia, contactless examinations of surfaces of various measured objects. To acquire the surface contour of an object to be examined, an object beam from a light source of the interferometer hits the surface at the area to be measured. The object beam reflected from the surface is supplied to a detector of the interferometer and, together with a reference beam, forms an interference pattern, from which the path length difference of the two beams may be derived. This measured path length difference of the two beams corresponds to the topography change of the surface.
In particular using a white light interferometer, in which the light source outputs short coherent radiation, it is also possible to scan the measured object using depth scanning. As explained in unpublished German Patent application No. DE 103 25 443.9, for example, the short coherent radiation is divided via a beam splitter into an object beam and a reference beam. The object surface to be measured is imaged via an objective on an image recorder, such as a CCD camera (“charge-coupled device” camera), and has the reference wave formed by the reference beam superimposed on it. Depth scanning may be performed by moving a reference mirror reflecting the reference beam or the objective in relation to the measuring device. During movement of the object, the image plane of the object and the reference plane are in the same plane. During depth scanning, the object remains fixed in the field of view of the CCD camera, and the object is only moved in the depth axis in relation to the reference plane. In this way, technical surfaces having a depth resolution in the range of a few nanometers may be measured. The technical basis of this measurement method is also found in the article “Three-dimensional sensing of rough surfaces by coherence radar” (T. Dresel, G. Häusler, H. Venzke, Appl. Opt. 31 (7), p. 919-925, 1992).
It is often desirable to image more than one side of the measured object. In practice, for example, to determine the thickness of a measured object, such as the thickness of a disk, both sides of the disk are measured using object beams through a mirror system in a special-purpose objective. For this purpose, the object beams are deflected perpendicularly onto the two sides of the disk by two deflection mirrors. The beams reflected from the sides of the disk are supplied to the image recorder and used together with the reference beams for recording the correlogram and finally analyzed to obtain the height data. As already explained above, a depth scan is performed during the measurement, or, in other words, the sides of the measured object to be measured are moved through the focal plane of the camera. Alternatively, it is also possible to change the focal plane of the camera through electrically controllable lenses or lens systems.
A disadvantage of the mirror system described, however, is that the relative position of the measured object in relation to the two deflection mirrors cannot be checked. Optimum positioning of the measured object is provided when the two beams, which are directed to the first or second side of the measured object, respectively, and, having been reflected therefrom, are supplied to the image recorder, must cover an equally long light path in each case. For this purpose, the measured object must be positioned precisely in the middle between the two deflection mirrors. If the measured object is positioned offset because of a lack of checking capability, i.e., the light paths of the two beams have different lengths, this results in a longer measurement duration in comparison to optimum positioning.
If the measured object is incorrectly positioned by approximately 20 μm, an additional scanning path of 2×20 μm=40 μm must be covered. A typical measuring speed in scanning interferometers is approximately 5 μm/second. This results in an additional measurement time of 8 seconds. When checking all measured objects using cycle times of a few seconds, this time dimension is unacceptable, particularly in industrial manufacturing.